1. Field of the Invention
This invention relates to a process for producing a halogen-containing polycarbonate resin. More particularly, it relates to a process for producing a halogen-containing polycarbonate resin in which washability of an organic solvent solution of the halogen-containing polycarbonate resin obtained by the phosgene method is improved. The polycarbonate resin which has a reduced content of impurities, is inflammable and has excellent melt flowability.
2. The Prior Art
Known processes of producing a halogen-containing polycarbonate resin by the phosgene method include:
(1) A process in which a mixture of a halogenated bisphenol and bisphenol A, and phosgene are reacted to directly give a resin (U.S. Pat. No. 3,062,781); and
(2) A process in which a polychloroformate (oligomer) prepared from bisphenol A and a halogenated bisphenol A are reacted to obtain a resin of high molecular weight (Japanese Patent Publication No. 38-7147).
In the preparation of halogenated polycarbonate resins by such a phosgene method, it is the general practice to wash an organic solvent solution of the resin obtained by polymerization with an aqueous medium such as water or an aqueous acid solution so as to purify the resin, and to pulverize and concentrate the resin. When it is desired to produce, by the known method, a polycarbonate resin containing a halogen in an amount required for the flame retarding purpose or in a relatively large amount, say, above 4 wt%, the organic solvent solution of a resin obtained by the polymerization reaction is much more difficult to wash and to pulverize and concentrate, as compared with a case where bisphenol A alone is used as a starting material. The thus obtained polycarbonate resin having a large content of halogen has a further disadvantage that it is lower in melt fluidity than a polycarbonate resin obtained from a starting material of bisphenol A alone.
For instance, when a solution, in methylene chloride, of 10 wt% of a polycarbonate resin having a bromine content of above 4 wt%, which has been obtained by reacting tetrabromobisphenol A and bisphenol A with a polycarbonate oligomer by the foregoing process (2), is washed on an industrial scale, several disadvantages result: (1) An emulsion is formed and thus an organic solvent solution of the resin is not separated from the aqueous washing; (2) Even though separation takes place, a large amount of a dirt layer is formed and thus the washing efficiency is too poor to remove impurities satisfactorily, so that the resulting resin is poor in color tone and also in thermal stability; and (3) upon concentrating and powdering, a viscous gel-like material is produced which is hard to powder, making it difficult to produce a particulate material on an industrial scale.